1. Field of Invention
The invention relates to a logic circuit and, in particular, to an auxiliary processing unit composed of a logic circuit for programmable logic controllers.
2. Related Art
Currently, automated equipment is often controlled using a programmable logic controller (PLC, or simply called programmable controller). This is particularly true for routine controls. The control behavior of the PLC is designed using the Ladder Diagram software. The functions of the PLC include basic logic operations, timing, and counting. It is further equipped with memory to satisfy industrial procedural controls. The data processing commands are also executed by the central processing unit (CPU) of the PLC. With the need of processing analog signals, there are analog-to-digital (A/D) converter modules available. With the need of communications and networking, one can also add a networking module.
Basically, the PLC can be considered as a microcomputer with a special interface. All data processing jobs are performed within the CPU. An input module obtains the status of an external controlled system. A program determines which drivers on an output module should be activated to drive the controlled system. The PLC is essentially a small computer specifically designed for a routine control system. After a user writes a program and stores it in a storage device, the CPU follows the control logic defined in the code to monitor and process the input signal from buttons, sensors, or threshold switches. After logic operations, output signals are sent to an external load, such as a relay, indicator, and motor. Sometimes, if necessary, the output signal can be fed back as the input signal to control other output devices.
Most conventional PLC's use a single chip along with a simple logic IC to implement all functions. It is acceptable for normal routine controls and applications that do not require high execution speeds. However, there still exist problems in command execution speed, counting frequency, and pulse output frequency that affect the efficiency of the system.
Besides, after connecting to an expanded machine, the single chip has to control the input/output (I/O) timing and data by itself. When connecting to an application specific integrated circuit (ASIC), the program coding and executing seem to be very inefficient.
In the function of counting, the highest counting frequency is around 10 kHz. The highest counting frequency lowers when many sets of counting are performed simultaneously. Moreover, if one needs to output different styles of pulses, the highest counting frequency also decreases.
Currently, there are two major solutions for the above problems. One is to use a better single chip. This does not only increases the cost, the firmware designer has to learn new tools too. The other method is to use additional hardware to expand its functions.
To have wider applications, the PLC has a high requirement for its execution efficiency, including the program execution speed, the basic pulse I/O function, and its expansion abilities. Therefore, using a CPU to implement all functions is not only in efficient but also unable to satisfy many of the user's demands.